This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to RF excitation pulses as used in MRI.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and for medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field the radio frequency signals resulting from the precession of the spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are then combined to produce a volumetric image of the nuclear spin density of the body.
MRI signals for recording an image of an object are obtained by placing the object in a magnetic field, applying magnetic gradients for slice selection, applying a magnetic excitation pulse to tilt nuclei spins in the desired slice, and then detecting MRI signals emitted from the tilted nuclei spins.
In auto-triggered MR angiography of intra-cranial arteries, arteries are monitored with a real-time pulse sequence and the signal statistics are used to automatically detect the arrival of a bolus of contrast agent, triggering an instantaneous switch to a high-resolution 3D acquisition (See Farb et al.,“Intracranial arteriovenous malformations: real-time auto-triggered elliptic centric-ordered 3D gadolinium-enhanced MR angiography—initial assessment.”, Radiology 220:244–251, 2001.) The signal variation due to in-flow is a source of uncertainty. RF saturation pulses can be applied adjacent to the imaging slice prior to each data acquisition to reduce in-flow enhancement, but this typically requires a longer repetition time (TR) and a loss in temporal resolution. Temporal resolution is an issue in applications such as intracranial MR angiography because the transit time from the arterial to the venous system is short and precise timing minimizes venous contamination.
RF saturation pulses are used in MRI to suppress unwanted signals. These pulses are typically implemented as a 90-degree, slice-selective excitation followed by a dephasing “crusher” gradient pulse. However, this takes several milliseconds, increasing the minimum repetition time (TR) for rapid pulse sequences, and decreasing flexibility for other, longer TR sequences.